1. Field of the Invention
The present disclosure relates to liquid crystal display devices, and more particularly to method and device for driving local dimming in a liquid crystal display device, which can suppress brightness drop while moderating gradation concentration caused by data compensation at the time of local dimming.
2. Discussion of the Related Art
Currently, as image display devices, flat display devices, such as liquid crystal display device LCD, plasma display panel PDP, and organic light emitting diode OLED display device, are used, mostly.
The liquid crystal display device is provided with a liquid crystal panel which displays an image with a pixel matrix which uses electric and optical characteristics of liquid crystals having anisotropy in refractive index and dielectric, a driving circuit for driving the liquid crystal panel, and a backlight unit for directing a light to the liquid crystal panel. Each of pixels of the liquid crystal display device produces gradation by controlling transmissivity of a light transmitting the liquid crystal panel and a polarizing plate from the backlight unit by varying orientation of the liquid crystals in response to a data signal.
In the liquid crystal display device, brightness of each of the pixels is the multiplication of brightness of the backlight unit to the light transmissivity of the liquid crystals in response to a data signal. In order to improve a contrast ratio and reduce power consumption, the liquid crystal display device uses backlight dimming in which a received image is analyzed to adjust a dimming value for controlling backlight brightness and compensating data. For an example, the backlight unit dimming method for reducing the power consumption reduces the backlight unit brightness by reducing the dimming value and improves the brightness by compensating the data.
Currently, as the backlight unit, an LED backlight unit is used, which uses light emitting diodes LED having advantages of high brightness and low power consumption compared to a related art lamp. Since the LED backlight unit enables local brightness control, the LED backlight unit can be driven by a local dimming method in which the LED backlight unit is divided into a plurality of light emitting blocks for controlling the brightness block by block. In the local dimming, an image data is analyzed block by block to determine a local dimming value, and controls the brightness of the LED backlight unit block by block as well as compensates the image data block by block, to improve the contrast ratio further and to reduce the power consumption more.
However, though the local dimming compensates the brightness by increasing the data as much as reduced brightness of the backlight unit by the local dimming, since there is a limit in data increase, resulting to compensate all of the pixels in a high gradation (bright) region with the same threshold value, the local dimming has a problem of occurrence of gradation concentration.
In order to moderate the gradation concentration, a gradation roll-off method has been suggested, in which the data in the high gradation region is adjusted to be dark throughout the region. However, since the related art gradation roll-off method is always applied to the high gradation region regardless of an image characteristic, though the related art gradation roll-off method has an advantage of increasing a range of displayable gradation values in an image which has much gradation concentration, the related art gradation roll-off method has a drawback of drop of the brightness in an image which has no gradation concentration.